Heretofore, fertilizers for growing rice, for example, lime fertilizers, magnesia fertilizers, silicate fertilizers, nitrolime, phosphate fertilizers, potassium salts, calcium perphosphate, compound fertilizers (N, P.sub.2 O.sub.5), ammonium sulfate, ammonium phosphate, urea, ammonium urate and the like were used.
It is also known that rice and wheat absorb high amounts of silicate. The ash of rice straw contains ca.80% SiO.sub.2 and the ash of wheat straw contains ca.70% SiO.sub.2. When these gramineous plants lack silicates, silificication of epidermal cells does not occur. As a result, stalks become weak and easily fall down. Moreover, the plants become vulnerable to disease such as blast.
It was 1955 when silicate became known as a fertilizer component. Since then, by-product slag from iron industries or non-iron metal industries came to be used with the name of SICA fertilizer (Keikaru fertilizer). This slag also contains CaO and MgO.
Since many SICA fertilizers consist of alkaline calcium silicate and magnesium silicate, they play a role in neutralizing acidic soils. When such SICA fertilizers are used, the effect of the fertilizer appears slowly, since the decomposition of the alkaline silicate salt is needed for silicic acid (SiO.sub.2.nH.sub.2 O) to be generated. Also, the influence of calcium residues and the change in the pH values are not negligible after the decomposition.
As aforementioned, nowadays, when silicic acid is to be fed to rice or wheat, the SICA fertilizer is used. But in that case, CaO, MgO and other impurities are strewn at the same time. As a result, problems such as changes in pH values of soil or unforeseeable troubles due to impurities can occur.